A. Testosterone Metabolism in Men.
Testosterone is the major circulating androgen in men. More than 95% of the 6-7 mg of testosterone produced per day is secreted by the approximately 500 million Leydig cells in the testes. Two hormones produced by the pituitary gland, luteinizing hormone (“LH”) and follicle stimulating hormone (“FSH”), are required for the development and maintenance of testicular function.
The most important hormone for the regulation of Leydig cell number and function is LH. In eugonadal men, LH secretion from the pituitary is inhibited through a negative-feedback pathway by increased concentrations of testosterone through the inhibition of the release of gonadotropin-releasing hormone (“GRH”) by the hypothalamus. FSH promotes spermatogenesis and is essential for the normal maturation of sperm. FSH secretion from the pituitary normally is inhibited through a negative-feedback pathway by increased testosterone concentrations.
Testosterone is responsible primarily for the development and maintenance of secondary sex characteristics in men. In the body, circulating testosterone is metabolized to various 17-keto steroids through two different pathways. Testosterone can be metabolized to dihydrotestosterone (“DHT”) by the enzyme 5α-reductase. There are two forms of 5α-reductase in the body: one form is found predominately in the liver and non-genital skin while another form is found in the urogenital tract of the male and the genital skin of both sexes. Testosterone can also be metabolized to estradiol (“E2”) by an aromatase enzyme complex found in the liver, fat, and the testes.
Testosterone circulates in the blood 98% bound to protein. In men, approximately 40% of the binding is to the high-affinity sex hormone binding globulin (“SHBG”). The remaining 60% is bound weakly to albumin. Thus, a number of measurements for testosterone are available from clinical laboratories. The term “free” testosterone as used herein refers to the fraction of testosterone in the blood that is not bound to protein. The term “total testosterone” or “testosterone” as used herein means the free testosterone plus protein-bound testosterone. The term “bioavailable testosterone” as used herein refers to the non-SHBG bound testosterone and includes testosterone weakly bound to albumin.
The conversion of testosterone to DHT is important in many respects. For example, DHT binds with greater affinity to SHBG than does testosterone. In addition, in many tissues, the activity of testosterone depends on the reduction to DHT, which binds to cytosol receptor proteins. The steroid-receptor complex is then transported to the nucleus where it initiates transcription and cellular changes related to androgen action. DHT is also thought to lower prostate volume and inhibit tumor development in the prostate. Thus, given the importance of DHT and testosterone in normal body functioning, researchers frequently assess and report androgen concentrations in patients as total androgen (“DHT+T”) or as a ratio of DHT to testosterone (“DHT/T ratio”).
The following table from the UCLA-Harbor Medical Center summarizes the hormone concentrations in normal adult men range:
TABLE 1Hormone Levels in Normal MenHormoneNormal RangeTestosterone298 to 1043 ng/dLFree testosterone3.5 to 17.9 ng/dLDHT31 to 193 ng/dLDHT/T Ratio0.052 to 0.33DHT + T372 to 1349 ng/dLSHBG10.8 to 46.6 nmol/LFSH1.0 to 6.9 mlU/mLLH1.0 to 8.1 mlU/mLE217.1 to 46.1 pg/mL
There is considerable variation in the half-life of testosterone reported in the literature, ranging from 10 to 100 minutes. Researchers do agree, however, that circulating testosterone has a diurnal variation in normal young men. Maximum levels occur at approximately 6:00 to 8:00 a.m. with levels declining throughout the day. Characteristic profiles have a maximum testosterone level of 720 ng/dL and a minimum level of 430 ng/dL. The physiological significance of this diurnal cycle, if any, however, is not clear.
B. Hypogonadal Men and Current Treatments for Hypogonadism.
Male hypogonadism results from a variety of patho-physiological conditions in which testosterone concentration is diminished below the normal range. The hypogonadic condition is sometimes linked with a number of physiological changes, such as diminished interest in sex, impotence, reduced lean body mass, decreased bone density, lowered mood, and energy levels.
Researchers generally classify hypogonadism into one of three types. Primary hypogonadism includes the testicular failure due to congenital or acquired anorchia, XYY Syndrome, XX males, Noonan's Syndrome, gonadal dysgenesis, Leydig cell tumors, maldescended testes, varicocele, Sertoli-Cell-Only Syndrome, cryptorchidism, bilateral torsion, vanishing testis syndrome, orchiectomy, Klinefelter's Syndrome, chemotherapy, toxic damage from alcohol or heavy metals, and general disease (renal failure, liver cirrhosis, diabetes, myotonia dystrophica). Patients with primary hypogonadism show an intact feedback mechanism in that the low serum testosterone concentrations are associated with high FSH and LH concentrations. However, because of testicular or other failures, the high LH concentrations are not effective at stimulating testosterone production.
Secondary hypogonadism involves an idiopathic gonadotropin or LH-releasing hormone deficiency. This type of hypogonadism includes Kallman's Syndrome, Prader-Labhart-Willi's Syndrome, Laurence-Moon-Biedl's Syndrome, pituitary insufficiency/adenomas, Pasqualini's Syndrome, hemochromatosis, hyperprolactinemia, or pituitary-hypothalamic injury from tumors, trauma, radiation, or obesity. Because patients with secondary hypogonadism do not demonstrate an intact feedback pathway, the lower testosterone concentrations are not associated with increased LH or FSH levels. Thus, these men have low testosterone serum levels but have gonadotropins in the normal to low range.
Third, hypogonadism may be age-related. Men experience a slow but continuous decline in average serum testosterone after approximately age 20 to 30 years. Researchers estimate that the decline is about 1-2% per year. Cross-sectional studies in men have found that the mean testosterone value at age 80 years is approximately 75% of that at age 30 years. Because the serum concentration of SHBG increases as men age, the fall in bioavailable and free testosterone is even greater than the fall in total testosterone. Researchers have estimated that approximately 50% of healthy men between the ages of 50 and 70 have levels of bioavailable testosterone that are below the lower normal limit. Moreover, as men age, the circadian rhythm of testosterone concentration is often muted, dampened, or completely lost. The major problem with aging appears to be within the hypothalamic-pituitary unit. For example, researchers have found that with aging, LH levels do not increase despite the low testosterone levels. Regardless of the cause, these untreated testosterone deficiencies in older men may lead to a variety of physiological changes, including sexual dysfunction, decreased libido, loss of muscle mass, decreased bone density, depressed mood, and decreased cognitive function. The net result is geriatric hypogonadism, or what is commonly referred to as “male menopause.” Today, hypogonadism is the most common hormone deficiency in men, affecting 5 in every 1,000 men. At present, it is estimated that only five percent of the estimated four to five million American men of all ages with hypogonadism currently receive testosterone replacement therapy. Thus, for years, researchers have investigated methods of delivering testosterone to men. These methods include intramuscular injections (43%), oral replacement (24%), pellet implants (23%), and transdermal patches (10%). A summary of these methods is shown in Table 2.
TABLE 2Mode of Application and Dosage of Various Testosterone PreparationsPreparationRoute Of ApplicationFull Substitution DoseIn Clinical UseTestosterone enanthateIntramuscular injection200-25.0 g every 2-3 weeksTestosterone cypionateIntramuscular injection200 mg every 2 weeksTestosterone undecanoateOral2-4 capsules at 40 mg per dayTransdermal testosterone patchScrotal skin1 membrane per dayTransdermal testosterone patchNon-scrotal skin1 or 2 systems per dayTestosterone implantsImplantation under the3-6 implants of 200 mg every 6abdominal skinmonthsUnder DevelopmentTestosterone cyclodextrinSublingual2.5-5.0 mg twice dailyTestosterone undecanoateIntramuscular injection1000 mg every 8-10 weeksTestosterone buciclateIntramuscular injection1000 mg every 12-16 weeksTestosterone microspheresIntramuscular injection315 mg for 11 weeksObsolete17α-MethyltestosteroneOral25-5.0 g per dayFluoxymesteroneSublingual10-25 mg per dayOral10-20 mg per day
As discussed below, all of the testosterone replacement methods currently employed suffer from one or more drawbacks, such as undesirable pharmacokinetic profiles or skin irritation. Thus, although the need for an effective testosterone replacement methodology has existed for decades, an alternative replacement therapy that overcomes these problems has never been developed. The present invention is directed to a 1% testosterone hydroalcoholic gel that overcomes the problems associated with current testosterone replacement methods.
1. Subdermal Pellet Implants.
Subdermal implants have been used as a method of testosterone replacement since the 1940s. The implant is produced by melting crystalline testosterone into a cylindrical form. Today, pellet implants are manufactured to contain either 100 mg (length 6 mm, surface area 1172) or 200 mg of testosterone (length 12 mm, surface area 202 mm2) Patients receive dosages ranging from 100 to 1,200 mg, depending on the individual's requirements. The implants are inserted subcutaneously either by using a trocar and cannula or by open surgery into an area where there is relatively little movement. Frequently, the implant is placed in the lower abdominal wall or the buttock. Insertion is made under local anesthesia, and the wound is closed with an adhesive dressing or a fine suture.
Implants have several major drawbacks. First, implants require a surgical procedure which many hypogonadal men simply do not wish to endure. Second, implant therapy includes a risk of extrusion (8.5%), bleeding (2.3%), or infection (0.6%). Scarring is also a risk. Perhaps most important, the pharmacokinetic profile of testosterone pellet implant therapy fails to provide men with a suitable consistent testosterone level. In general, subdermal testosterone implants produce supra-physiologically high serum testosterone levels which slowly decline so that before the next injection subnormally low levels of testosterone are reached. For example, in one recent pharmacokinetic study, hypogonadal patients who received six implants (1,200-mg testosterone) showed an initial short-lived burst release of testosterone within the first two days after application. A stable plateau was then maintained over then next two months (day 2: 1,015 ng/dL; day 63: 990 ng/dL). Thereafter, the testosterone levels declined to baseline by day 300. DHT serum concentrations also rose significantly above the baseline, peaking at about 63 days after implementation and greatly exceeding the upper limit of the normal range. From day 21 to day 189, the DHT/T ratio was significantly increased. The pharmacokinetic profiles for testosterone, DHT, and DHT/T in this study are shown in FIG. 1. See “Jockenhovel et al., Pharmacokinetics and Pharmacodynamics of Subcutaneous Testosterone Implants in Hypogonadal Men,” 45 Clinical Endocrinology 61-71 (1996). Other studies involving implants have reported similar undesirable pharmacokinetic profiles.
2. Injection of Testosterone Esters.
Since the 1950s, researchers have experimented with the intermuscular depot injection of testosterone esters (such as enanthate, cypionate) to increase testosterone serum levels in hypogonadal men. More recent studies have involved injection of testosterone buciclate or testosterone undecanoate in an oil-based vehicle. Other researchers have injected testosterone microcapsule formulations.
Testosterone ester injection treatments suffer from many problems. Patients receiving injection therapy often complain that the delivery mechanism is painful and causes local skin reactions. In addition, testosterone microcapsule treatment requires two simultaneous intramuscular injections of a relatively large volume, which may be difficult to administer due to the high viscosity of the solution and the tendency to block the needle. Other men generally find testosterone injection therapy inconvenient because injection usually requires the patient to visit his physician every two to three weeks.
Equally important, injection-based testosterone replacement treatments still create an undesirable pharmacokinetic profile. The profile generally shows a supra-physiologic testosterone concentration during the first 24 to 48 hours followed by a gradual fall often to sub-physiologic levels over the next few weeks. These high serum testosterone levels, paralleled by increases in E2, are also considered the reason for acne and gynecomastia occurring in some patients, and for polycythaemia, occasionally encountered especially in older patients using injectable testosterone esters. In the case of testosterone buciclate injections, the treatment barely provides normal androgen serum levels and the maximal increase of serum testosterone over baseline does not exceed 172 ng/dL (6 nmol/dL) on average. Because libido, potency, mood, and energy are thought to fluctuate with the serum testosterone level, testosterone injections have largely been unsuccessful in influencing these variables. Thus, testosterone injection remains an undesirable testosterone replacement treatment method.
3. Oral/Sublingual/Buccal Preparations of Androgens.
In the 1970s, researchers began using oral, sublingual, or buccal preparations of androgens (such as fluoxymesterone, 17α-methyl-testosterone, or testosterone undecanoate) as a means for testosterone replacement. More recently, researchers have experimented with the sublingual administration of testosterone-hydroxypropyl-beta-cyclodextrin inclusion complexes. Predictably, both fluoxymesterone and methyl testosterone are 17-alkylated and thus associated with liver toxicity. Because these substances must first pass through the liver, they also produce an unfavorable effect on serum lipid profile, increasing LDL and decreasing HDL, and carbohydrate metabolism. While testosterone undecanoate has preferential absorption through the intestinal lymphatics, it has not been approved in the United States.
The pharmacokinetic profiles for oral, sublingual, and buccal delivery mechanisms are also undesirable because patients are subjected to super-physiologic testosterone levels followed by a quick return to the baseline. For example, one recent testing of a buccal preparation showed that patients obtained a peak serum hormone levels within 30 minutes after administration, with a mean serum testosterone concentration of 2,688+/−147 ng/dL and a return to baseline in 4 to 6 hours. See Dobs et al., Pharmacokinetic Characteristics, Efficacy and Safety of Buccal Testosterone in Hypogonadal Males: A Pilot Study, 83 J. Clinical Endocrinology & Metabolism 33-39 (1998). To date, the ability of these testosterone delivery mechanisms to alter physiological parameters (such as muscle mass, muscle strength, bone resorption, urinary calcium excretion, or bone formation) is inconclusive. Likewise, researchers have postulated that super-physiologic testosterone levels may not have any extra beneficial impact on mood parameters such as anger, nervousness, and irritability.
4. Testosterone Transdermal Patches.
The most recent testosterone delivery systems have involved transdermal patches. Currently, there are three patches used in the market: TESTODERM® (testosterone transdermal), TESTODERM® TTS (testosterone transdermal), and ANDRODERM® (testosterone transdermal system).
a. TESTODERM® (Testosterone Transdermal).
TESTODERM® (testosterone transdermal, Alza Pharmaceuticals, Mountain View, Calif.) was the first testosterone-containing patch developed. The TESTODERM® (testosterone transdermal) patch is currently available in two sizes (40 or 60 cm2). The patch contains 10 or 15 mg of testosterone and delivers 4.0 mg or 6.0 mg of testosterone per day. TESTODERM® (testosterone transdermal) is placed on shaved scrotal skin, aided by application of heat for a few seconds from a hair dryer.
FIG. 2 shows a typical pharmacokinetic testosterone profile for both the 40 cm2 and 60 cm2 patch. Studies have also shown that after two to four weeks of continuous daily use, the average plasma concentration of DHT and DHT/T increased four to five times above normal. The high serum DHT levels are presumably caused by the increased metabolism of 5α-reductase in the scrotal skin.
Several problems are associated with the TESTODERM® (testosterone transdermal) patch. Not surprisingly, many men simply do not like the unpleasant experience of dry-shaving the scrotal hair for optimal contact. In addition, patients may not be able to wear close-fitting underwear when undergoing treatment. Men frequently experience dislodgment of the patch, usually with exercise or hot weather. In many instances, men experience itching and/or swelling in the scrotal area. Finally, in a number of patients, there is an inability to achieve adequate serum hormone levels.
b. TESTODERM® TTS (Testosterone Transdermal).
The most recently developed non-scrotal patch is TESTODERM® TTS (testosterone transdermal, Alza Pharmaceuticals, Mountain View, Calif.). It is an occlusive patch applied once daily to the arm, back, or upper buttocks. The system is comprised of a flexible backing of transparent polyester/ethylene-vinyl acetate copolymer film, a drug reservoir of testosterone, and an ethylene-vinyl acetate copolymer membrane coated with a layer of polyisobutylene adhesive formulation. A protective liner of silicone-coated polyester covers the adhesive surface.
Upon application, serum testosterone concentrations rise to a maximum at two to four hours and return toward baseline within two hours after system removal. Many men, however, are unable to obtain and/or sustain testosterone levels within the normal range. The pharmacokinetic parameters for testosterone concentrations are shown as follows:
TABLE 3TESTODERM ® TTS (testosterone transdermal) Testosterone ParametersParametersDay 1Day 5Cmax (ng/dL)482 ± 149473 ± 148Tmax (h)3.93.0Cmin (ng/dL)164 ± 104189 ± 86 Tmin (h)0  0  
The typical 24-hour steady state testosterone concentration achieved with TESTODERM® TTS (testosterone transdermal) patch is shown in FIG. 3.
Because of TESTODERM® (testosterone transdermal) patch is applied to the scrotal skin while the TESTODERM® TTS (testosterone transdermal) patch is applied to non-scrotal skin, the two patches provide different steady-state concentrations of the two major testosterone metabolites, DTH and E2:
TABLE 4Hormone Levels Using TESTODERM ® (testosterone transdermal)and TESTODERM ® TTS (testosterone transdermal)TESTODERM ®TESTODERM ®TTS (testosteroneHormonePlacebo(testosterone transdermal)transdermal)DHT (ng/dL)1113438E2 (pg/ml)3.81021.4
Likewise, in contrast to the scrotal patch, TESTODERM® TTS (testosterone transdermal) treatment creates a DHT/T ratio that is not different from that of a placebo treatment. Both systems, however, suffer from similar problems. In clinical studies, TESTODERM® TTS (testosterone transdermal) is associated with transient itching in 12% of patients, erythema in 3% of patients, and puritus in 2% of patients. Moreover, in one 14-day study, 42% of patients reported three or more detachments, 33% of which occurred during exercise.
c. ANDRODERM® (Testosterone Transdermal System).
ANDRODERM® (testosterone transdermal system, Watson Laboratories, Inc., Corona, Calif.) is a testosterone-containing patch applied to non-scrotal skin. The circular patch has a total surface area of 37 cm.2 The patch consists of a liquid reservoir containing 12.2 mg of testosterone and a permeation-enhanced vehicle containing ethanol, water, monoglycerides, fatty acid esters, and gelling agents. The suggested dose of two patches, applied each night in a rotating manner on the back, abdomen, upper arm, or thigh, delivers 4.1 to 6.8 mg of testosterone.
The steady state pharmacokinetic profile of a clinical study involving ANDRODERM® (testosterone transdermal system) is shown in FIG. 4. In general, upon repeated application of the ANDRODERM® (testosterone transdermal system) patch, serum testosterone levels increase gradually for eight hours after each application and then remain at this plateau level for about another eight hours before declining.
In clinical trials, ANDRODERM® (testosterone transdermal system) is associated with skin irritation in about a third of the patients, and 10% to 15% of subjects have been reported to discontinue the treatment because of chronic skin irritation. Preapplication of corticosteroid cream at the site of application of ANDRODERM® (testosterone transdermal system) has been reported to decrease the incidence and severity of the skin irritation. A recent study, however, found that the incidence of skin reactions sufficiently noxious enough to interrupt therapy was as high as 52%. See Parker et al., Experience with Transdermal Testosterone Replacement in Hypogonadal Men, 50 Clinical Endocrinology (Oxf) 57-62 (1999). The study reported: Two-thirds of respondents found the ANDROPATCH® (testosterone transdermal) unsatisfactory. Patches were variously described as noisy, visually indiscrete, embarrassing, unpleasant to apply and remove, and generally to be socially unacceptable. They fell off in swimming pools and showers, attracted ribald comments from sporting partners, and left bald red marks over trunk and limbs. Dogs, wives, and children were distracted by noise of the patches with body movements. Those with poor mobility or manual dexterity (and several were over 70 years of age) found it difficult to remove packaging an apply patches dorsally.
d. Transdermal Patch Summary.
In sum, the transdermal patch generally offers an improved pharmacokinetic profile compared to other currently used testosterone delivery mechanisms. However, as discussed above, the clinical and survey data shows that all of these patches suffer from significant drawbacks, such as buritus, burn-like blisters, and erythema. Moreover, one recent study has concluded that the adverse effects associated with transdermal patch systems are “substantially higher” than reported in clinical trials. See Parker, supra. Thus, the transdermal patch still remains an inadequate testosterone replacement therapy alternative for most men.
5. DHT Gels.
Researchers have recently begun investigating the application of DHT to the skin in a transdermal gel. However, the pharmacokinetics of a DHT-gel is markedly different from that of a testosterone gel. Application of DHT-gel results in decreased serum testosterone, E2, LH, and FSH levels. Thus, DHT gels are not effective at increasing testosterone levels in hypogonadal men. Accordingly, there is a definite need for a testosterone formulation that safely and effectively provides an optimal and predictable pharmacokinetic profile.